The invention relates to an apparatus for regulating the gas/air ratio for a pre-mixing combustion device.
In order to minimise the emission of pollutants and to optimise the efficiency of combustion devices, efforts are made to maintain the gas/air ratio at an optimal point.
Systems with a pneumatic coupling and systems with an electronic coupling are known.
In the case of systems with a pneumatic coupling, the volumetric flow of the gas is maintained in a constant ratio to the volumetric flow of the air. This is effected by means of a pressure-controlled regulator in the gas pathway. A pressure difference is used as the control pressure to regulate the gas volumetric flow in direct dependence on the induced volumetric flow of the air. The control pressure for the regulator is produced in a mixing device or at an orifice in the air pathway.
The systems with a pneumatic coupling require no electrical or electronic components. Their mechanical construction is very simple and operates very rapidly. They need only a few moveable components. The mixture preparation with a pneumatic coupling has the advantage of reacting very rapidly to variations in pressure and volumetric flow in the combustion installation and maintaining the gas/air ratio constant within a large modulation range. These robust systems have thus proved themselves to be satisfactory in many installations.
Influence parameters, such as gas quality, air pressure and combustion air temperature, can, however, not be compensated for by these systems by control techniques and necessitate an adjustment to the installations. As a result of the liberalisation of the gas market, relatively large variations in gas quality are to be expected, which make constant adjustment necessary with these systems.
Furthermore, systems with a pneumatic coupling become imprecise with a large modulation range in the lower power range, particularly after a relatively long period of operation. In order to ensure the combustion quality over the long term, the minimum power which is set may thus not be, depending on the construction, less than a value of 15–20%.
In systems with an electronic coupling, the composition of the combustion gases in the flame or after the flame or the flame temperature are measured by a sensor. The sensor produces a signal, which is analysed by an electronic regulator. The latter controls a regulating member, which determines the volume of gas. These systems with an electronic coupling are slower to stabilise control as regards changes in the air volume as regards the control. The sensors which are used are sensitive and have a limited surface life. A satisfactory sensor signal is necessary for the reliable functioning of the combustion installation. Variations in the gas quality, the air pressure and the combustion air temperature can, however, be compensated for by the controller.